The subject of the present invention is a process and an apparatus for the continuous coating of at least one metal strip with a thin fluid film of crosslinkable polymer containing neither solvent nor diluent.
Thermally crosslinkable polymers such as, for example, thermosetting polymers, or physically crosslinkable polymers such as, for example, photocurable polymers, are known.
Various processes are known for applying a thermoplastic or thermosetting organic coating to a bare or coated metal strip.
The application of organic coatings such as, for example, liquid paints or varnishes is usually carried out by roller coating these liquid coatings in the state of a solution or of a dispersion in an aqueous or solvent medium.
To do this, the liquid coating is deposited on a metal strip by predosing the solution or dispersion using a system comprising two or three rollers and by transferring some or all of this liquid coating thus predosed onto an applicator roller in contact with the surface of the metal strip to be coated.
The transfer is performed either by friction of the applicator roller on the metal strip, the two surfaces in contact running in opposite directions, or by contact in the same direction.
An advantageous trend in the technology of continuous application of crosslinkable polymer coatings, such as thermosetting paints or varnishes for example, to a metal strip consists in depositing this coating without the use of a solvent or a diluent.
To apply such coatings, several techniques are known at the present time.
The first consists in applying the organic coating in the form of a powder.
Another technique for applying a liquid coating to a metal strip is known, this technique using a heating tank, usually called a melting kettle, provided in its lower part with an orifice from which the liquid polymer contained in the tank flows.
Placed below this tank are two parallel rolls in contact with each other and the metal strip to be coated moves along beneath these rolls.
The liquid polymer is poured into the nip of the rolls, then flows between the said rolls and is deposited on the metal strip.
However, this technique has drawbacks stemming from the fact that the polymer can be only slightly reactive on account of its relatively long storage time in the heating tank and from the fact that it does not allow the thickness of the coating film on the metal strip to be controlled and consequently does not allow a thin homogeneous coating to be obtained.
Another technique consists in using the extrusion of the organic coating in the fluid state and in applying this coating to a substrate by extrusion coating or by lamination.
It is common practice to apply a thin organic coating, particularly of thermoplastic polymers, by extrusion coating to flexible substrates, such as paper, plastic films, textiles or even thin metal substrates, such as packaging materials.
The molten coating is transferred by means of a rigid sheet die positioned in direct contact with the substrate and the thickness of this coating does not depend on the die aperture but only on the flow rate of molten material through the section of the said die and on the speed of the substrate.
This technique requires there to be perfect parallelism between the edges of the die and the substrate.
In addition, the pressure exerted by the die on the substrate derives only from the viscosity of the molten material so that any possibility of correcting the discrepancies in flatness of the substrate, by pressing the latter against the back-up roll, is very limited.
The technique of extrusion lamination of a uniform layer of fluid coating on a substrate uses the drawing beneath the die of a fluid sheet at the exit of a sheet die, this sheet then being pressed against the substrate with the aid, for example, of a cold roller or of a rotating bar, or else by an air knife or an electrostatic field.
In this case, the thickness of the fluid sheet is controlled by the flow rate of the material in the die section and by the speed of the substrate.
In order to prevent the fluid sheet from sticking on the pressing roller, the latter must have a perfectly smooth and cooled surface.
In addition, the pressing pressure must however be low enough to prevent the formation of a calendering bead and consequently, this mode of transfer does not make it possible to compensate for any thickness variations and discrepancies in flatness in the case of a rigid substrate.
This technique of applying the coating with the formation of a free strand at the exit of the extrusion die makes it possible to avoid the problems of coupling between the die and the rigid substrate, but it causes application instabilities if the length of the free strand fluctuates.
In general, in the various known techniques mentioned above, the continuous application of a thin organic coating to metal substrates is carried out with low contact pressures, insufficient to allow production of a thin uniform coating applied homogeneously to rigid substrates which may have flatness and thickness-heterogeneity discrepancies.
These various application techniques do not make it possible to compensate for the variations in thickness of the metal substrate, which variations consequently cause unacceptable fluctuations in the thickness of the coating, especially if the substrate is formed by a metal strip which exhibits significant surface roughness and/or corrugations of amplitude equal to or greater than the thickness of the coating to be produced on the said metal strip.
Moreover, these various application techniques do not make it possible to allow for variations in the width of the substrate nor variations in the transverse positioning of this substrate, so that the coating cannot be deposited uniformly over the entire width of the said substrate.
Finally, during application of the coating, air microbubbles may be trapped between the coating and the substrate, which is to the detriment of homogeneous application and to the surface appearance of this coating and which, in some applications, causes rapid corrosion of the substrate at these air microbubbles.
Moreover, EP-A-0,593,708 discloses an apparatus for coating a running metal strip in which this metal strip passes over a back-up roll and is in contact with an applicator roll at the surface of which a fluid coating agent is deposited.
The applicator roll is driven in rotation in an opposite direction to the direction in which the strip runs, and this roll is made of metal.
This apparatus has drawbacks.
This is because, given the nature of the material of which the applicator roll is made, this apparatus does not allow homogeneous pressure to be applied at all the points of application of the film to the metal strip between the two rollers when the surface of this metal strip is excess reliefs, that is to say exhibits roughness and/or corrugations which are equivalent to or greater than the thickness of the coating film to be deposited.
This is because the surface hardness of the applicator roller does not allow the pressure to be applied and distributed uniformly and locally to the strip and there is even the risk of the coating film spreading, to the detriment of the homogeneity of its
Consequently, the continuous application of a thin uniform coating of crosslinkable polymer to a metal strip therefore causes problems because this metal strip has flatness and thickness discrepancies as well as significant roughness and/or corrugations of amplitude equal to or greater than the thickness of the coating film to be deposited on the said strip, even when this strip is pressed with a high force against a uniform roll.
The object of the invention is to avoid these drawbacks by providing a process and an apparatus for the continuous coating of at least one metal strip with a thin fluid film of crosslinkable polymer containing neither solvent nor diluent, making it possible to obtain a coating of uniform thickness of a few microns applied homogeneously to this strip, while preventing air microbubbles from being trapped between the film and the metal strip and obviating discrepancies in the flatness and roughness of this strip as well as allowing application on part or all of the coating, despite the fluctuations in width and transverse positioning of this strip.
The subject of the invention is therefore a process for the continuous coating of at least one metal strip with a fluid film of crosslinkable polymer containing neither solvent nor diluent and with a thickness of less than that of the metal strip, characterized in that:
the metal strip is made to run continuously over at least one back-up roll;
the said metal strip is preheated to a temperature approximately equal to or greater than the temperature of the fluid film of crosslinkable polymer and equal to or greater than the softening temperature of this crosslinkable polymer;
a homogeneous and uniformly thick sheet of the said crosslinkable polymer is formed, by forced flow, on a pinch roll having a hard surface, the said crosslinkable polymer having a viscosity greater than 10 Pa.s under the conditions of formation of the said sheet, the said pinch roll being heated to a temperature approximately equal to or greater than the temperature of the said sheet and equal to or greater than the softened temperature of this crosslinkable polymer and being driven in rotation in the same direction as the back-up roll supporting the metal strip, and the film of crosslinkable polymer is formed with a uniform thickness approximately equal to the desired thickness;
an applicator element having a deformable surface is compressed between the pinch roll and the metal strip, the said applicator element being heated to a temperature approximately equal to or greater than the temperature of the pinch roll and being driven in rotation in the same direction as the back-up roll supporting the metal strip, in order to transfer this film firstly from the pinch roll onto the applicator element and secondly from the said applicator element onto the metal strip and to obtain a coating of homogeneous thickness.
The subject of the invention is also an apparatus for the continuous coating of at least one metal strip with a thin fluid film of crosslinkable polymer containing neither solvent nor diluent and with a thickness of less than that of the metal strip, characterized in that it comprises:
means for continuously driving the metal strip;
at least one back-up roll supporting the metal strip;
means for preheating the metal strip to a temperature equal to or greater than the temperature of the fluid film of crosslinkable polymer and equal to or greater than the softening temperature of this crosslinkable polymer;
means for forming, on a pinch roll having a hard surface, by forced flow, a homogeneous and uniformly thick sheet of the said crosslinkable polymer having a viscosity greater than 10 Pa.s under the conditions of formation of the said sheet, the said pinch roll forming the film of crosslinkable polymer with a uniform thickness approximately equal to the desired thickness and being heated to a temperature approximately equal to or greater than the temperature of the said sheet and equal to or greater than the softening temperature of this crosslinkable polymer and being driven in rotation in the same direction as the back-up roll supporting the metal strip;
and an applicator element having a deformable surface, the said applicator element being compressed between the take-up roll and the metal strip, heated to a temperature approximately equal to or greater than the temperature of the pinch roll and driven in rotation in the same direction as the back-up roll supporting the metal strip, in order to transfer the said film firstly from the pinch roll onto this applicator element and secondly from the said applicator element onto the metal strip and to obtain a coating of homogeneous thickness.